Sources of elemental fractionation and uncertainty during the analysis of semi-volatile metals in silicate glasses using LA-ICP-MS

Abstract

We investigate elemental fractionation and sources of analytical uncertainty during in situ determination of a range of semi-volatile trace metals (e.g., Cd, Sn, Pb, Zn, Cu, Mo) in silicate glasses using laser ablation-ICP-MS (LA-ICP-MS), and compare the performance of single-volume and two-volume ablation chambers. In a single-volume ablation chamber we document the differential response of volatile and refractory elements relative to ⁴³Ca at different ablation sites within the ablation chamber as a primary source of analytical uncertainty. This fractionation is unrelated to interaction between the laser pulse and solid material during progressive ablation, but does correlate with the local He velocity at the position of analysis. Evidence suggests that fractionation relates to differences in behaviour of refractory and volatile elements during condensation from the laser-induced plasma, and interaction between condensate and the carrier gas. The dependence of fractionation on local He flow regime results in relatively poor reproducibility in ⁴³Ca normalized ratios (up to ∼20%, 2 s) for a number of volatile metals as well as some with siderophile and chalcophile tendencies (e.g., B, Co, Cu, Zn, Mo, Ag, In, Sn, Sb, W, Pb). Fractionation of this type may be a major feature of many single-volume ablation chambers and also may occur in other instances where the He flow regime varies substantially with location in the ablation chamber. Analyses within a two-volume sample chamber, where the He flow rate at the site of ablation remains more uniform across the chamber, show no evidence for this style of elemental fractionation, and normalized ratios measured for volatile trace metals show reproducibilities for normalized ratios that are typically <10% (2 s).